Stigmatic and achromatic system for deflecting a particle beam

ABSTRACT

The present invention relates to magnetic deflection devices for beams of charged particles. The system according to the invention comprises four deflecting elements arranged in pairs symmetrically with respect to a plane perpendicular to the plane containing the path of the beam. The distance between the faces of the deflecting elements is adjusted so that the energy conjugates of the two intermediate deflecting element coincide with the energy foci of the terminal deflecting elements. The device is thus achromatic.

0 United States Patent [151 3,691,374 Leboutet [451 Sept. 12, 1972 [54] STIGMATIC AND ACHROMATIC 3,087,055 4/1963 Liebl ..250/4l.9

SYSTEM FOR DEFLECTING A PARTICLE BEAM FOREIGN PATENTS OR APPLICATIONS [72] Inventor; Hubert Lemma Paris, France 1,484,852 5/1967 France ..250/4l.9 [731 Assignw Thomson CSF Primary Examiner-William F. Lindquist 22 Filed; 25 1970 Attorney-Cushman, Darby & Cushman [21] Appl. No.: 66,849 57 ABSTRACT The present invention relates to magnetic deflection [30] Foreign Amman Priority devices for beams of charged particles.

Sept. 10, 1969 France ..6930797 The System according to the invention comprises four deflecting elements arranged in pairs symmetrically 8 "250/495 B ag/ 8 with respectthto a flllanef pfh peracdicular 1t: th; Plane O n o I q a n e e e n u u a no. e c I c n n a I e o e m e [58] held of searchmzsolflg 3 g 6 between the faces of the deflecting elements is adjusted so that the energy conjugates of the two intermediate deflecting element coincide with the energy [56] References Cited foci of the terminal deflecting elements. The device is UNITED STATES PATENTS thus achromatic- 3,405,363 10/1968 Brown ..328/230 8 Claims, 7 Drawing Figures PATENTEU SEP 12 I972 SHEET 1 BF 3 PATENTEU SEP 12' m2 SHEEI 3 [1F 3 STIGMATIC AND ACI-IROMATIC SYSTEM FOR DEFLECTING A PARTICLE BEAM The present invention relates to devices for directing and focussing a beam of particles such as electrons, i.e. fundamental material particles. Such devices are to be found in particular in the field of particle accelerators for nuclear research purposes and for industrial and medical gammaradiography applications. Frequently, recourse is had to the action of a magnetic field created between pole-pieces, the latter determining an air-gap crossed by said beam.

If the magnetic field used has a direction perpendicular to the paths of the particles, they are deflected. The

- deflection angle, for a given magnetic field strength,

depends upon the masses and the velocities of the particles and, in the general case, depends upon the product of these two quantities; i.e. the momenta and, as a consequence, of the energies of the particles. Generally speaking, a single electromagnet is not capable of producing identical deflections of particules of different energies, in other words it is not achromatic in the-energy sense".

By combination of several such electromagnets, separated from one another by drift spaces in which the particles are not subjected to any magnetic field, it is possible to produce deflections of a beam within a plane, in order to direct it in a predetermined direction and onto a predetermined point at the same time,

precise geometric, electronic and energetic characteristics can be obtained that the system is limited to small deflection angles, smaller than .60.

The, present invention relates to a system by which the same results can be achieved for larger deflection angles, for example lying between 90 and 360. The system essentially being of such design that a straight path at entry to the system is converted at the exit therefrom into another straight path whose position and direction are independent of the energies of said particles.

An object of the present invention is a stigmatic and achromatic magnetic deflection device for deflecting, within a plane, a beam of charged particles, said deflection device comprising first and second identical magnetic deflectors and intermediate deflector means crossed by said beam and positioned symmetricallywith respect to a further plane perpendicular to said plane said further plane being a plane of symmetry of said intermediate deflector means said beam being subjected to magnetic field vectors parallel to each other and perpendicular to said plane the direction of said field vectors being selected for bending said beam, along a mean path crossing successively the first magnetic deflector, said intermediate deflector means and the second magnetic deflector which is identical to said first said mean path having rectilinear segments separated by substantially curvilinear segments, said rectilinear segments including on both sides of the first deflector a first and a second rectilinear segment at an angle a and a third and a fourth rectilinear segment at said angle a said second and on both sides of the second deflector third rectilinear segments situated on both sides of said intermediate deflection being at an angle 20 said intermediate deflector means and said first and second magnetic deflectors having input and output faces said input and output faces of said interr r Ta.n 0 Tan 0:

For a better understanding of the invention and to show how the same may be carried into effect reference will be made to the drawing accompanying the ensuing description and in which FIG. 1 illustrates an electromagnet forming'a part of the device in accordance with the invention.

FIG. 2 illustrates a device in accordance with the invention.

FIG. 3 illustrates schematically the magneto-optical properties of the device of FIG. 2.

FIG. 4 illustrates another device in accordance with the invention.

FIG. 5 illustrates a device in which respectively the input and output faces of the first and second magnetic deflectors are no longer perpendicular to the beam.

FIG. 6 illustrates the same device as FIG. 5, this time however the two intermediate magnets being separated.

FIG. 7 illustrates a practical embodiment of the device in accordance with the invention.

Typical particle paths have been illustrated in FIGS. 2, 3, 4, 5 and 6, namely the path A for particles of medium energy and central position, path B for particles of medium energy but with incident path parallel to A, and path C for particles of central position but having energies differing from the mean value.

FIG. 1 schematically illustrates a deflection electromagnet and is designed to indicate that known property of this type of apparatus, upon which the invention is based.

The electromagnet is composed of a yoke plate 1 with its two arms 2, two electrical coils 3, and two polepieces 4 which define an air-gap 5, crossed by a particle beam for the purpose of deflection. The input faces 6 and output faces 7 of these pole-pieces have an intersection 8 at which the center of curvature of the mean path 9 of the deflected particles, is located.

Under these circumstances, the physical laws which govern the electron-optical system, show that the electromagnet has an infinite number of pairs of particular points, such as M and M, located in the mean path plane of the air-gap and symmetrically in relation to the plane of symmetry of the magnet perpendicular to said mean path plane, such that particles falling with a certain energy band or energy spread" and passing through one such point, also pass through the other. These points are the energy conjugates" of the deflec tion system. The positions of these pairs of particular points, depend upon the characteristic operating parameters of the electromagnet, being linked by relationships including factors such as the angle of input and output faces of the electro magnet pole-pieces, the respective intervals of the points in question from the faces located opposite them, the mean radius of the paths, the distance of the extreme paths of the beam from the mean path, and the energy spread in the beam.

On the other hand, particles falling within a certain energy band and entering its air-gap along a mean path are concentrated by the magnetic field of the electromagnet, output of said electromagnet at a point which we shall term here the energyfocus.

In addition, as in conventional (light) optics, we can define the geometric focus of a beam of particles of given energy as the point of intersection of the projection of the trajectories of those of the exiting particles whose entry trajectories are parallel to the center axis of the beam.

Since magnetic deflection systems do not generally exhibit the same geometric optical properties in the plane of deflection as they do in the plane perpendicular thereto, we shall define hereinafter by the term geometric focus, the focus relating to the plane of deflection, the system in accordance with theinvention, and its variant embodiments, being convergent or afocal in the plane perpendicular to this plane, but never divergent.

FIG. 2 illustrates by way of example a-device in accordance with the invention.

The magnetic deflection system comprises a first magnetic deflector which is a first electromagnet l and a second identical magnetic deflector which is a second electromagnet 11 such as those illustrated in FIG. 1, having a sectoral pole-pieces at an angle a and an intermediate deflector means formed by an electromagnet 12 having sectoral pole-pieces at an angle 20. These electromagnets are arranged in succession in such a way that one of the energy conjugate points of one of the electromagnet coincides with one of the energy focii of another of the electromagnets in the system.

FIG. 3 schematically illustrates the paths of the particles through the devicedescribed hereinbefore. Calculation shows that it is possible in this fashion to produce a system which is totally insensitive to differences in particle energy, i.e. by analogy with conventional optics where the wavelength of light is replaced by the particle energy, is non-dispersive" or achromatic in the energy sense". Another way of putting it is to say that the system is afocal in the energy sense. The respective magnetic fiels are parallel vectors perpendicular to the plane of the figure, the air-gaps being symmetrically positioned with respect to the plane containing the mean path of the beam. The arrangement of the first and second magnetic deflectors is such that the assembly has a plane of symmetry P, which is the plane of symmetry of the intermediate magnetic deflector means represented by the electromagnet 12. The input face 6 and output face 7 of pole pieces 4 of the first electromagnet l0 and second electromagnet 11 are at an angle a and the input and output faces 6 and 7 of the electromagnet 12, are at an angle 20 the latter faces are respectively parallel to output face 7 of electromagnet 10 and input face 6 of the electromagnet 11.

The direction of magnetic field H is indicated by an arrow.

Considering now the mean path A, the rectilinear segments 1, II, III and IV are separated by curvilinear segments. The beam enters the sectoral pole piece perpendicularly to the input face 6 of the sectoral pole piece 4 and emerges from said sectoral pole piece perpendicularly to the output face 7. The angle of the sectoral pole piece of electromagnet 10 is the angle a formed by the rectilinear segment I and II of said mean path A. Similarly the angle of segments III and IV and of the sectoral pole piece of electromagnet 11 has a same value a.

FIG. 3 schematically illustrates the trajectories of the particles through the device described hereinbefore.

It is possible here to consider the intermediate electromagnet of angle 26, as made of two elementary intermediate electromagnets of angle 0 having a common interface.

If the distance between the output face of said first magnetic deflector and the input face of said intermediate deflector is selected to be equal to Tan 0 Tan 0:

then the conditions of the invention are met and the energy focus of the first electromagnet will coincide with the energy conjugate M of the intermediate electromagnet, situated on the same side.

Simultaneously, the geometric focus of the first electromagnet will coincide at H with the geometric focus of half the intermediate magnetic deflector of angle 26.

It may be noticed that all paths of particles crossing the plane of symmetry P of the system, are parallel. This can be put another way by saying that tat this location the energy foci and geometric foci, are projected to infinity.

Experience and theory show that, at the device exit, a beam is obtained whose position and direction are independant of the energy of particles a rectilinear particle path characterized by its position and its slope in relation to the mean path, and by its energy difference with respect to the energy of particles of the mean path, will, at exit, exhibit the same geometric characteristics irrespective of said difference.

Only the modification introduced by the deflection system, is equivalent to an advance or forward translation of the trajectories by a length geometrically equal T Sin ZlH-sin a Sin 0! FIG. 4 illustrates a variant embodiment of the device in accordance with the invention in which the intermediate deflector means if formed by two electromagnets of angle 0, substituted to the intermediate electromagnet having an angle 20 this possibility already having been referred to hereinbefore, said electromagnets having the mutually opposite faces of their polepieces at equal distances from the plane of symmetry P.

The introduction of this length or drift space" at this point of the particle path, in no way modifies the properties of the system since at this point the paths crossing the plane of symmetry are parallel.

It is exclusively the effect already indicated, of forward translation and experienced by the geometrical characteristics of position and slope of the beam, which is modified, this time by the effect of rearward translation. For a specific value of the length of the drift spaces equal to r Sin 20+ sin 2a min Q4 the forward translation is exactly compensated. The exiting beam is identical to the entering beam in terms of position and slope, whatever the energy.

FIG. 5 illustrates another variant embodiment of the device in accordance with the invention in which the input face 6 of said first electromagnet l0 and the output face 7 of said second electromagnet 11, make an angle y different from zero. The intermediate deflector means being one electromagnet 12 at an angle 20.

If the angle y is such that the angle of the planes of the input and output faces of said first electromagnet .is smaller than the initial angle a, the effect produced in the plane of the drawing is equivalent to that of a divergent lens and the geometric focus thus displaces towards the intermediate deflector means 12.

For a given value of the angle y, such as the geometric focus 1-! is located in the plane of symmetry P of the overall system. On the other hand, since the energy focus is still at infinity for paths crossing this plane, it is possible to locate at this point in the plane of symmetry a device for analyzing the particle energy, such as a slot moving in said plane and associated with a collecting element.

FIG. 6 illustrates an advantageous variant embodiment of the device shown in FIG. 5, in which the intermediate magnetic deflector of angle 20 is remplaced by two electromagnets at an angle 0 separated by a .distance 2L.

For a specific value ofy such as Tan Y: 2 sin a L (Sin 20+sin 2a) -2 7 sin 20 the geometric focus of the half-device is still in the plane of symmetry P thereof, and the advantages already referred to concerning the possibility of energy analysis of the beam are secured, this time, however, with the supplementary benefit of a wider freedom of choice in the arrangement of the necessary apparatus, this freedom resulting from the introduction of a drift space of length 2L, between the two intermediate electromagnets.

it may be advantageous to create the desired divergence effect by giving the angle y a smaller value, in the order of for example 60, and by supplementing the effect through the use of two magnetic lenses 20 and 21 of conventional structure arranged beyond the terminal faces of the end magnets, as shown in FIG. 6.

FIG. 7 illustrates an embodiment of the device in accordance with the invention, viewed in section in the plane of symmetry of the air-gap.

.The device comprises a yoke plate 22 and two yoke arms 23, common to the three electromagnets described hereinbefore, these latter being distinguished simply by their pole-pieces 24, 25 and 26.

The terminal faces 27 and 28 of the intermediate magnet have a cylindrical curvature produced in accordance with a conventional method in order to correct the spherical aberration of the system.

Finally, a magnetic coil 29, with its leads 30 and 31, surrounds the set of pole-pieces.

What I claim is 1. A stigmatic and achromatic magnetic deflection device for deflecting, within a plane, a beam of charged particles, said deflection device comprising first and second identical magnetic deflectors and intermediate deflector means crossed by said beam and positioned symmetrically with respect to a further plane perpendicular to said plane said further plane being a plane of symmetry of said intermediate deflector means said beam being subjected to magnetic field vectors parallel to each other and perpendicular to said plane the direction of said field vectors being selected for bending said beam, along a mean path crossing successively the first magnetic deflector, said intermediate deflector means and the second magnetic deflector said mean path having rectilinear segments separated by substantially curvilinear segments, said rectilinear segments including on both sides of the first deflector, a first and second rectilinear segment and, on both sides of the second deflector, a third and a fourth rectilinear segment: said second and third rectilinear segments situated on both sides of said intermediate deflector means said first and second magnetic deflectors each deflecting the beam through a curvilinear segment of angle a and said intermediate deflector means deflecting the beam through a curvilinear segment of angle 200 said intermediate deflector means and said first and second magnetic deflectors having respective input and output faces said input and output faces of said intermediate deflector means being respectively parallel to the output face of said first deflector and the input face of said second deflector; the rectilinear segments of said mean path being normal to the respective faces of the first and second deflectors and the intermediate deflector means with which the rectilinear segments intersect; the mean path of said beam having within said intermediate deflector means a mean radius of curvature r the input and output faces of said intermediate deflector means being respectively separated from the output face of said first deflector and the input face of said second deflector by a distance substantially equal to 7' 7' pilanjfi iane 2. A device as claimed in claim 1 wherein said first and second identical deflectors are electromagnets having sectoral pole-pieces symmetrically positioned with respect to said further plane, said pole-pieces determining air-gaps crossed by said beam.

3. A device as claimed in claim 1 wherein said intermediate deflector means comprise a third deflector having sectoral pole-pieces at an angle 20, the input and output faces of said third deflector being symmetrically positioned with respect to said further plane.

4. A device as claimed in claim 1 wherein said intermediate deflector means comprise a third and a fourth magnetic deflector having sectoral pole-pieces symmetrically positioned with respect to said further plane the input and output faces of said pole-pieces being at an angle 0 the input face of said fourth deflector being parallel to the output face of said third deflector.

5. A device as claimed in claim 4, wherein the distance separating the input face of said fourth magnetic deflector from the output face of said third deflector is substantially equal to sin 20+sin 2a i 2 sin a 6. A device as claimed in claim wherein said mean path intersects the input face of said first magnetic deflector and the output face of said second magnetic deflector at an angle y to the plane of the faces the tangent of said angle 7 being substantially equal to 2 sin a Tan (Sin 20+sin 2a) 2 Lf /1- s i}1 2 0 7. A device as claimed in claim 6 wherein in front of the input face of said first deflector and beyond the out- 2 sin 1x Sin 2(9-l-sin 2oz 

1. A stigmatic and achromatic magnetic deflection device for deflecting, within a plane, a beam of charged particles, said deflection device comprising : first and second identical magnetic deflectors and intermediate deflector means crossed by said beam and positioned symmetrically with respect to a further plane perpendicular to said plane ; said further plane being a plane of symmetry of said intermediate deflector means ; said beam being subjected to magnetic field vectors parallel to each other and perpendicular to said plane ; the direction of said field vectors being selected for bending said beam, along a mean path crossing successively the first magnetic deflector, said intermediate deflector means and the second magnetic deflector ; said mean path having rectilinear segments separated by substantially curvilinear segments, said rectilinear segments including on both sides of the first deflector, a first and second rectilinear segment and, on both sides of the second deflector, a third and a fourth rectilinear segment: said second and third rectilinear segments situated on both sides of said intermediate deflector means ; said first and second magnetic deflectors each deflecting the beam through a curvilinear segment of angle Alpha and said intermediate deflector means deflecting the beam through a curvilinear segment of angle 2 theta 0 ; said intermediate deflector means and said first and second magnetic deflectors having respective input and output faces ; said input and output faces of said intermediate deflector means being respectively parallel to the output face of said first deflector and the input face of said second deflector ; the rectilinear segments of said mean path being normal to the respective faces of the first and second deflectors and the intermediate deflector means with which the rectilinear segments intersect; the mean path of said beam having within said intermediate deflector means a mean radius of curvature r ; the input and output faces of said intermediate deflector means being respectively separated from the output face of said first deflector and the input face of said second deflector by a distance substantially equal to
 2. A device as claimed in claim 1 wherein said first and second identical deflectors are electromagnets having sectoral pole-pieces symmetrically positioned with respect to said further plane, said pole-pieces determining air-gaps crossed by said beam.
 3. A device as claimed in claim 1 wherein said intermediate deflector means comprise a third deflector having sectoral pole-pieces at an angle 2 theta , the input and output faces of said third deflector being symmetrically positioned with respect to said further plane.
 4. A device as claimed in claim 1 wherein said intermediate deflector means comprise a third and a fourth magnetic deflector having sectoral pole-pieces symmetrically positioned with respect to said further plane ; the input and output faces of said pole-piecEs being at an angle theta ; the input face of said fourth deflector being parallel to the output face of said third deflector.
 5. A device as claimed in claim 4, wherein the distance separating the input face of said fourth magnetic deflector from the output face of said third deflector is substantially equal to
 6. A device as claimed in claim 5 wherein said mean path intersects the input face of said first magnetic deflector and the output face of said second magnetic deflector at an angle gamma to the plane of the faces the tangent of said angle gamma being substantially equal to :
 7. A device as claimed in claim 6 wherein in front of the input face of said first deflector and beyond the output face of said second deflector are positioned magnetic elements for controlling the beam divergence.
 8. A device as claimed in claim 1, wherein said mean path intersects the input face of said first magnetic deflector and the output face of said second magnetic deflector at an angle gamma to the plane of the faces: the tangent of said angle gamma being substantially equal to 